1. Field of the Invention
The invention relates to a portable machine tool, particularly a portable table saw having an outer casing, an electric drive motor in the casing, a connecting device for connecting the motor to a power supply network with a low system frequency, and a tool driven by the drive motor.
The invention relates to a mobile machine tool which is to be understood as neither a firmly installed, nor a hand-held machine tool, rather a machine tool which sits on a table or a stand in order to perform the machining of the workpiece; however, as far as size and weight go, it is constructed so that it can be transported from place to place, advisably able to be carried by one or two people.
2. Description of Related Art
A mobile machine tool of the type to which the invention is directed is known (German Utility Model No. U 297 21 232). This is only one example of a type of portable machine tool, such machine tools having been known in the art for decades.
Weight is especially important for mobile machine tools. This is substantially defined by the weight of the electric drive motor, at least when considerable shaft power is required, as, for example, in table circular saws. In order to keep the weight of this type of mobile machine tool as low as possible, universal motors are used. These run with a high rotational speed, build small and have a comparably low weight. A shaft power of 1 kW at 18,000 RPM means a motor weight of 1.5 to 2.0 kg and over-all dimensions of 0.75 1 for a universal motor.
For machine tools, particularly table circular saws, universal motors are therefore problematic in practice because they are sensitive to wear due to their required collector ring and required brushes. Wear is increased even more, especially for table circular saws, through the dust-filled surroundings.
The object of the invention is to develop a more wear-resistant mobile machine tool, particularly a table circular saw, of the sort in question at a low as possible weight.
The above-mentioned object is met with a mobile machine tool described in the preamble by the features of the characterizing part of claim 1. According to the invention, a substantially maintenance-free asynchronous motor is used as an electric drive motor. The use of an asynchronous motor for machine tools is, naturally, fundamentally known. Until now, however, the weight and space conditions for asynchronous motors were seen as hindering so that asynchronous motors have only been used reluctantly for mobile machine tools. At the least, the weight of the mobile machine tools was, then, comparably high. This is because an asynchronous motor is tied to the system frequency with its rotational speed. The rotational speed is slightly less than the synchronous rotational speed which is determined by the quotient of the system frequency divided by the number of pole pairs. At a normal system frequency of 50 Hz, a shaft power of 1 kW at 3000 RPM re-quires a motor weight of 8.0 to 10.0 kg and a dimension requirement of the asynchronous motor of 1.5 to 2.0 liters. The portion of iron and coils of the asynchronous motor are decisive for the weight and volume of the asynchronous motor.
The asynchronous motor is operated with a considerably higher operating frequency by the inventively suggested combination of the asynchronous motor with a frequency converter. Through this, a substantially higher rotational speed is reached. At the given shaft power, a substantially smaller asynchronous motor is needed which has a substantially lower weight and less volume. The weight of the mobile machine tool which is equipped with the asynchronous motor having a frequency converter is thus considerably lowered since the weight of the frequency converter is barely of consequence. The total reduction of weight and volume is estimated at about 50% in comparison to the asynchronous motors used until now not having a frequency converter.
At similar electrotechnical starting positions, a synchronous motor which is tied precisely to the rotational speed of the rotary field, i.e. shows no slip-page, can also be used in place of an asynchronous motor.
The invention is described below in greater detail with reference to the accompanying drawings which show a single embodiment of the invention.